This invention generally relates to any micromachined structure in which it is desirable to isolate the effect of a mechanical stimulus on a front surface to the mechanical forces transmitted to bulk substrate. The invention is effective for both surface and bulk micromachining.
Recently semiconductor processes have been used to manufacture ultrasonic transducers of a type known as micromachined ultrasonic transducers (MUTs), which may be of the capacitive (cMUT) or piezoelectric (pMUT) variety. cMUTs are tiny diaphragm-like devices with electrodes that convert the sound vibration of a received ultrasound signal into a modulated capacitance. For transmission the capacitive charge is modulated to vibrate the diaphragm of the device and thereby transmit a sound wave.
One advantage of MUTs is that they can be made using semiconductor fabrication processes, such as microfabrication processes grouped under the heading “micromachining”. As explained in U.S. Pat. No. 6,359,367:                Micromachining is the formation of microscopic structures using a combination or subset of (A) Patterning tools (generally lithography such as projection-aligners or wafer-steppers), and (B) Deposition tools such as PVD (physical vapor deposition), CVD (chemical vapor deposition), LPCVD (low-pressure chemical vapor deposition), PECVD (plasma chemical vapor deposition), and (C) Etching tools such as wet-chemical etching, plasma-etching, ion-milling, sputter-etching or laser-etching. Micromachining is typically performed on substrates or wafers made of silicon, glass, sapphire or ceramic. Such substrates or wafers are generally very flat and smooth and have lateral dimensions in inches. They are usually processed as groups in cassettes as they travel from process tool to process tool. Each substrate can advantageously (but not necessarily) incorporate numerous copies of the product. There are two generic types of micromachining . . . 1) Bulk micromachining wherein the wafer or substrate has large portions of its thickness sculptured, and 2) Surface micromachining wherein the sculpturing is generally limited to the surface, and particularly to thin deposited films on the surface. The micromachining definition used herein includes the use of conventional or known micromachinable materials including silicon, sapphire, glass materials of all types, polymers (such as polyimide), polysilicon, silicon nitride, silicon oxynitride, thin film metals such as aluminum alloys, copper alloys and tungsten, spin-on-glasses (SOGs), implantable or diffused dopants and grown films such as silicon oxides and nitrides.The same definition of micromachining is adopted herein. The systems resulting from such micromachining processes are typically referred to as “micromachined electro-mechanical systems (MEMS).        
Conventional cMUTs resemble tiny drums that are “beat” electrostatically. The drumhead vibrates to both emit and receive ultrasonic waves. A cMUT probe consists of an array of many elements, each element comprising a respective multiplicity of individual cMUT cells.
A typical cMUT cell comprises a thin membrane (made, e.g., of silicon or silicon nitride) with an overlying metal electrode, suspended over a cavity (usually evacuated) formed over or in a silicon substrate. A bottom electrode is formed in or on the silicon substrate. Groups of cMUT cells may be electrically connected by hard wiring the top electrodes to each other. The driving force for the deflection of the membrane is the electrostatic attraction between the top and bottom electrodes when a voltage is impressed across them. If an alternating voltage drives the membrane, significant ultrasound generation results. Conversely, if the membrane is biased appropriately and subjected to incoming ultrasonic waves, significant detection currents are generated. Typical thicknesses of the membrane and the cavity gap are on the order of 0.5 micron. The lateral dimensions of the cMUT cell range from 100 to 30 microns for cMUT array operating frequencies of 2 to 15 MHz, respectively.
Most cMUTs are comprised of many small drumhead membranes tiled together. Typically, the membranes are supported by rigid walls around and between the individual cMUT cells. This rigid support structure between the membranes reduces the effective area of the transducer array and may contribute to unwanted structural resonances and crosstalk between transducer elements. The deflection of the drumhead is non-uniform, largest in the center and zero at the edges.
The transduction performance of each cMUT cell depends on the distance between the electrodes, the compliance of the supports, and the stiffness of the membrane (as well as factors such as the density and Poisson's ratio of the membrane). With stiff supports and a flexible membrane, the structure behaves as a drumhead, i.e., a traditional cMUT. There is a need for new cMUT structures that improve upon the performance of the conventional cMUT array.